1. Field of the invention
The present invention concerns an ionization cell for mass spectrometers.
In particular, the invention applies to mass spectrometers in which the heated electrical filament emitting electrons is replaced by a cold cathode of the micropoint type.
2. Description of the Prior Art
The advantages of a cold cathode over a tungsten filament heated to 1,800.degree. C. are well known:
the very high energy efficiency, which is practically 100%, each electron emitted having been taken from the emitting source in a ratio 1/1, unlike the tungsten filament that has to be heated with a high current for it to be able to emit electrons by a thermo-electronic effect; the orders of magnitudes of the powers employed are 10 W for a heated filament compared to 0.2 W for a cold source, PA1 the rapid reaction of the device, both on turning it on and on turning it off: in the case of sudden air entry, the system can be deactivated instantaneously, unlike a tungsten filament that will burn because of its thermal inertia; this rapid reaction additionally makes it feasible to cut off the power supply to the device when the instrument is not in measuring mode and to turn it on again to carry out a measurement, PA1 the directionality of the emitted beam: the electrons are emitted perpendicularly to the surface of the micropoint array, unlike a filament in which the electrons are emitted in all directions, and PA1 the absence of heat dissipation: the device emitting electrons by the field effect does not generate any heat and consequently does not disturb the operation of the temperature-sensitive detection pre-amplifiers.
However, reliability and operational capability are not assured at pressures in the order of 10.sup.-4 mbar.
At this pressure and above, the micropoint type cold cathode is degraded because of the excessively high number of ions formed between the cathode and the anode, constituting an ionization cage. The positive ions formed between the cathode and the ionization cage return to the negative cathode.